1. Field of the Invention
The present invention relates to a jet mill in which a dynamic classifier is integrated, and to a method for using the same.
2. Brief Description of the Prior Art
Jet mills have long been known as well suited for the production of extremely finely ground products (fine material). For instance, as early as 1935, the U.S. Pat. No. 2,032,827 B1 describes the construction of a cylindrical jet mill.
Later, such mills were used as a basis for developing so-called injector jet mills, in which the material to be ground that is to be comminuted is accelerated in a guiding tube, the injector, by a high-speed stream of gas and is then comminuted on impact, either against a baffle plate, or by interparticle collision (U.S. Pat. No. 1,847,009 B1).
It is disadvantageous for such a mill that, on account of the poor classification within the mill chamber, small amounts of coarse grain get into the fine material. In products for the coatings industry, even minute amounts of coarse grain have adverse effects on the use of these materials. For the purposes of the application, “coarse grain” means particles which have a particle size d(97) of >5 μm. “Particle size d(97)” means the particle size in μm below which 97% of all measured particles are in the volume distribution. If d(97)>5 μm, the pigment is of micronized quality.
In patent application DE 1159744 A1, it is attempted to counteract this disadvantage by combining the grinding unit with a dynamic air classifier. The air classifying pre-classification has the effect that the air classifier is in a zone where there is little coarse material. However, it is problematical here that the air flow rates between the air classifier and the mill cannot be synchronized well. During operation with the usual volumetric flows of propelling gas and grinding gas, the radial flow velocities against the air classifier are too great, or in the case of an air classifier of a larger size, the grinding zone gets too close to the air classifier. The required product qualities therefore cannot be achieved with this technology.
U.S. Pat. No. 2,237,091 B1 describes a so-called oval tube jet mill, in which a different type of combination of jet mill with static classification is obtained by separating the classifying zone and the grinding chamber. In these mills described, a deflecting type of separation is used as the classifying effect. In the known prior art, a static classifier is not capable in principle of adequately separating coarse grain.
DE 3730597 A1 and DE 2092626 A1 describe the combination of a static spiral air classifier and an oval jet mill. However, spiral air classifiers show the disadvantages of static classification. The required product qualities likewise cannot be achieved with this technology.
In a so-called fluidized-bed counter jet mill, the aforementioned disadvantages are overcome by the classifying chamber and the grinding chamber being physically separated, and an externally driven dynamic classifier, for example in the form of a paddle air classifier, being used as the classifier (DE 2040519 A1). The required product qualities likewise cannot be achieved with this technology.
The object of the present invention was therefore to provide a jet mill which makes it possible to remove ground material (54) with a particle size d(97) of ≦5 μm, preferably ≦3 μm after grinding as fine material.